Dutch Elm Disease

Although it doesn't seem to be a big problem yet, Dutch elm disease has been found in North Texas. American elm and all other native trees are susceptible.

DLD is a vascular fungal disease that is the same genus as oak wilt Cerastocystis spp. The name of DLD has recently been changed to Ophiostoma spp. for some reason. Both the oak and elm diseases are spread by beetles and root graphs.

Those people comfortable with the toxic chemical approach recommend spraying insecticides to attempt to control the beetle, trenching between live trees to break the root graphs, removing dead and seriously infected trees, pruning out infected limbs in the crown and injecting fungicides into the root flares. It's admitted that trees with the disease in the roots can't be saved, only those where the infected areas can be pruned out of the crown. The problem with the above approach is that it doesn't address the cause or the real problem and does nothing to prevent the disease from returning, assuming that it is actually stopped in the first place.

The real culprit is stressed, susceptible trees resulting from monoculture overplanting, unhealthy soil, excessive or ill-advised fertilization and other outside influences. The solution is the Sick Tree Treatment or some other form of soil and root system improvement.

The advantages to this approach are substantial. It not only functions as a preventative - it is also curative if the roots aren't too far gone. It is much more cost effective and even if the trees in question die, money isn't wasted because soil improvement has been done to help future plantings. Planting a greater variety of trees is also an important part of the solution.

The disease is spread in North America by two species of bark beetles Family: Curculionidae, The native elm bark beetle, Hylurgopinus rufipes, and the European Elm Bark Beetle, Scolytus multistriatus. In Europe, Scolytus multistriatus acts as vector for infection, it is much less effective than the Large Elm Bark Beetle Scolytus scolytus.

In an attempt to block the fungus from spreading further, the tree reacts to the presence of the fungus by plugging its own xylem tissue with gum and tyloses, bladder-like extensions of the xylem cell wall. As the xylem (one of the two types of vascular tissue produced by the vascular cambium, the other being the phloem), delivers water and nutrients to the rest of the plant, these plugs prevent them from traveling up the trunk of the tree, eventually killing it.

The first symptom of infection is usually an upper branch of the tree with leaves starting to wither and yellow in summer, months before the normal autumnal leaf shedding. This progressively spreads to the rest of the tree, with further dieback of branches. Eventually, the roots die, starved of nutrients from the leaves.

Often, not all the roots die: the roots may put up small suckers. These may grow up for some years into small elm trees, but after a decade or so the new trunks become large enough to support the bark beetles, and with their inevitable arrival the fungus returns, and the new tree dies.

I do not recommend injecting fungicides into trees and I don’t recommend removing trees that are near sick trees. The chemical injection hurts the tree, wastes money and doesn’t address the real problem. This procedure has been pushed by Texas A&M and the Texas Forest Service for many years and I have yet had anyone report that the fungicide injections have ever saved a single infected tree.

Biodiversity and soil health are the best deterrents. The disease is supposedly spread in the spring by a small beetle called nitidulid. There are probably many other possible vectors. Some people advise painting pruning cuts of live oak and red oak in the spring. I’m not so sure this is important. If you do, use Lac Balsam or natural shellac.

Dutch Elm Disease

• Initially affects individual branches OR affects lower crown nearest root graft.
• Leaves wilt and turn yellow, then brown.
• Symptoms often observed in early summer, but may be exhibited any time of the growing season.
• Brown streaking in sapwood.
• No discoloration in inner bark.
• No wintergreen odor.

• Affects the entire crown.
• Leaves turn yellow and may drop early.
• Symptoms visible from July to September.
• No discoloration in sapwood.
• Tan discoloration of inner bark.
• Wintergreen odor in inner bark.

• Damage initially observed on single branches, and spreads to entire crown; oldest leaves affected first.
• Leaves brown along margin, with a yellow halo.
• Symptoms appear in summer and early fall.
• No discoloration in sapwood.
• No discoloration of inner bark.
• No wintergreen odor.

Q.  I recently had to cut down my 80 year old lacebark elm because it had Dutch Elm disease. Now that I have the stumps removed I want to plant another elm. Will the new tree planted in the same place get the disease? Am I better off to plant a different type of tree? I was thinking about a red oak or Chinese pistachio. - A.E., Dallas

A.  It was probably cotton root rot instead of Dutch Elm disease. Lacebark elms have been pretty well proven to be a bad choice in soil that has had cotton grown on it in the past. No problem planting in the same place but I would switch to Chinese pistachio, shantung maple or one of the natives like Texas red oak, Texas ash, native pecan, bur oak or chinkapin oak.

 

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